1. Field of the Invention
The present invention relates to an image forming apparatus of an electrophotographic type and a method of forming an image.
2. Description of the Related Art
At present, an image forming apparatus of an electrophotographic type is utilized in a printer section for a laser printer, a copying machine, etc. Generally, a background image forming apparatus includes a photosensitive drum capable of rotating freely which is employed as a photosensitive body.
On an endless circumferential surface of the photosensitive drum capable of freely rotating, there are arranged, in order, a charger employed as a charging unit, a laser scanner as an exposing unit, a developing unit, and a transfer charger as a transfer unit opposed to the circumferential surface of the photosensitive drum. Furthermore, the apparatus is also provided with a paper conveying mechanism for conveying paper to be printed. A conveying path for conveying the paper is formed such that the paper passes through a gap between the photosensitive drum and the transfer charger.
In the case of forming an image by use of such an image forming apparatus, the rotating circumferential surface of the photosensitive drum is charged by the charger, and an electrostatic latent image is formed on the charged circumferential surface of the photosensitive drum by optical scanning of the laser scanner. The electrostatic latent image thus formed is developed with toner by the developing unit. The paper conveying mechanism conveys the paper to be printed in synchronism with such operations as mentioned above, and thereby the transfer charger electrostatically absorbs the toner image formed on the circumferential surface of the photosensitive body, and transfers the toner image onto the paper to be printed. According to the above-mentioned image forming apparatus, the transfer charger transfers the toner image on the photosensitive drum onto the paper.
Furthermore, there exists another image forming apparatus provided with an intermediate transfer unit. In such an image forming apparatus, for instance, the intermediate transfer unit includes an endless transfer belt. The transfer belt is suspended by plural guide rollers so as to circulate freely thereon. In such an apparatus, the toner image on the circumferential surface of the photosensitive drum is electrostatically absorbed onto the circumferential surface of the transfer belt. The toner image on the circumferential surface of the transfer belt is also electrostatically absorbed by another separate transfer charger, and thereby the toner image is transferred onto the paper.
Such a structure provided with the intermediate transfer unit is generally utilized for a color image forming apparatus. In such an image forming apparatus, a plurality of developing units are provided, and respective color toners are contained in the developing units respectively. When a color image is formed, the photosensitive drum is circulated repeatedly and each color toner image is formed one by one, and the respective color images are superposed in order on the circumferential surface of the transfer belt, and thereby the full-color toner image is formed. Finally, the color image thus completed on the circumferential surface of the transfer belt is transferred one time onto the surface of the paper.
Furthermore, in the above-mentioned various sorts of image forming apparatuses, there exists an apparatus having a photosensitive belt, another apparatus having an intermediate transfer unit provided with a transfer drum as a transfer body, and a combination thereof.
In all of the above-mentioned various sorts of image forming apparatuses, electric potential is created between the photosensitive body and the transfer unit, and thereby the toner image on the surface of the photosensitive body is electrostatically absorbed onto the circumferential surface of the transfer unit. Consequently, a transfer voltage of the transfer unit exerts an influence on the transfer efficiency thereof.
For instance, if a transfer voltage of the transfer unit is not sufficient, and thereby the electric potential difference between the photosensitive body and the transfer unit becomes insufficient, the toner cannot move preferably from the photosensitive body to the transfer unit. Therefore, the quality of the image is lowered and a burden on a toner cleaner increases inevitably. However, in the case of excessively increasing (raising) the transfer voltage of the transfer unit for preventing the above problems, electric power is unnecessarily consumed. Furthermore, as a result of the excessive voltage of the transfer unit, the toner is charged to an inverse polarity and is scattered around. Furthermore, the toner moves before the photosensitive body and the transfer unit are opposed to each other. Thereby, image quality is deteriorated.
In such a situation, in an image forming apparatus, the transfer conditions of a transfer unit are set in consideration of the above-mentioned matters. However, an optimum transfer condition of the transfer unit varies due to operational environments of the image forming device and time-elapsing variations of the respective portions. In order to cope with such problems, a temperature sensor and a humidity sensor can be disposed in the interior of the image forming apparatus and the transfer conditions of the transfer unit can be adjusted in accordance with the detected temperature and humidify. Furthermore, it may also be possible to forecast the time-elapsing variations of the photosensitive body and the transfer unit and to adjust the transfer conditions of the transfer unit time-elapsingly in accordance with the results of the above forecasting.
However, the above-mentioned two technologies can cope with only one of the environmental variations and the time elapsing variations. Although it is possible to employ a combination of these technologies, since the structure for realizing that may become complicated and the respective errors may be superposed (multiplied), both of these technologies are not practical. Furthermore, although the above-mentioned technologies take into consideration the environmental variations and the time-elapsing variations, both of these technologies cannot cope with manufacturing errors of a transfer unit or a transfer body exerting a prominent influence on transfer efficiency.